-( Story related, cannot be missed )-
Gumbald will have a trusty gun to change his attack type and can summon more aid if he has none (so keep one of them alive if you can endure the damage). Focus on trying to shock the minions with BMO so it can deal massive damage on Gumbald, or you can try to bunk him directly. He is affected by Sleep if you want to try and take some time recovering and dealing with the threats first. Compared to the Fire Giant, he is incredibly underpowered and you might not have a challenge with him unless you rushed through the game.